This invention is directed to a novel method for the preparation of a Group VIB metal promoted single phase crystalline vanadium(IV)bis(metaphosphate) catalyst which provides good yields of and selectivities to maleic anhydride by the oxidation of unsaturated aliphatic hydrocarbons.
Prior art processes are known for the preparation of phosphorus-vanadium-oxygen catalyst systems which include other metals or metal compounds as potential yield improving co-catalysts such as molybdenum, copper, and uranium oxides or other oxides incorporated into the catalyst system during preparation. These catalysts systems however, as well as other catalysts showing some activity for the production of maleic anhydride have generally proven to be unsatisfactory for commercial application and leave a lot to be desired since the yield and selectivity to maleic anhydride is usually low.
Phosphorus-vanadium-oxygen catalysts are usually prepared by reducing vanadium pentoxide to vanadium(IV) in water or an organic solvent with a suitable reducing acid or agent. A source of phosphorus, usually phosphoric acid, is mixed with the vanadium solution to produce a catalyst precursor which is heated to give the production catalyst. Co-catalysts are usually incorporated into the catalyst system during the solution stage of preparation and by such process there is a risk of also reducing the co-catalyst or added metal compound. The present invention avoids this problem.
U.S. Pat. No. 3,907,835 discloses the preparation of a catalyst comprising an admixture of vanadium, uranium, phosphorus and oxygen for the gas phase preparation of maleic anhydride.
U.S. Pat. No. 3,904,652 describes a solution or "reflux" method for the preparation of a phosphorus-vanadium-oxygen complex catalyst containing one or more activators selected from zinc, copper, bismuth or lithium intimately combined therewith.
U.S. Pat. No. 3,366,648 describes a solution method employing reducing agents to form a vanadium-phosphorus catalyst complex along with a phosphorus stabilizer of an alkali metal (Group 1a metal) compound useful for the preparation of maleic anhydride.
There is no known prior art which describes the preparation of a Group VIB metal, i.e., chromium, molybdenum or tungsten promoted crystalline VO(PO.sub.3).sub.2 hydrocarbon oxidation catalyst employing a liquid phase reaction of vanadyl sulfate, a group VIB metal oxide, acetic anhydride and phosphorus pentoxide followed by a thermal treatment with the liberation of gases to form a catalyst precursor.